In the well-known medical procedure of hemodialysis, blood is collected from a vascular access of a patient, passing through a well-known arterial hemodialysis set by means of a blood pump, to the hemodialyzer. The blood passes through hemodialyzer into a well-known venous hemodialysis set, by which the blood is conveyed back to the vascular access of the patient. Pediatric flow rates are as low as 20 ml/min while adult flow rates are as much as 650 ml/min. This entire set-up is known as an extracorporeal circuit.
The venous hemodialysis set has a well-known venous blood chamber, a so-called drip chamber that is typically 80-120 mm long and 17-25 mm outer diameter, designed to contain typically 7-35 ml of blood as a flow-through reservoir and operated such that blood does not completely fill the drip chamber. The chamber top has a blood inlet tube connection port in axial orientation communicating through the top to a blood inlet downspout in the main cavity of the chamber. The downspout length may vary such that the downspout outlet is either above the blood level (i.e. in the airspace), or beneath the blood level. Blood exits the venous chamber at the bottom by an outlet communicating axially through the bottom to a blood outlet tube connection port. A filter is placed within the chamber just above the blood outlet.
The venous drip chamber serves a number of functions:
1. Air bubbles contained in the incoming blood flow are forced by their buoyancy and reduction in velocity to rise to the air space. Flow out of the chamber is thereby degassed.
2. Blood pressure can be measured via a pressure monitor tube communicating through the chamber top into the air space.
3. Air or foam is also prevented from exiting the chamber by an ultrasonic air/foam detector attached to the outside of the drip chamber or the outside of the tubing just below the venous chamber. The detector stops the bloodpump and causes a clamp to close below the venous drip chamber if air or foam is sensed.
4. Medicament tube access ports or injection sites are located on the chamber top, communicating with the air space.
5. Clots, etc. are prevented from leaving the chamber by the presence of a filter adjacent the exit of the chamber.
Currently, venous chambers are injection molded of flexible or rigid plastic, typically comprising a gently conical shape with an open end which is closed with a solvent-sealed top cap, after a plastic filter has been inserted through the open end and placed into snap-fit relation with the chamber near the bottom end. Snap-fit is required because, typically, the filter material is not compatible for solvent bonding with the chamber material. Thus it is retained by detents on the inside of the chamber. This is an imperfect solution since:
1. Such a filter can occasionally come loose (when blood pressure expands the flexible chamber) and/or
2. The space between the filter attachment ring and the chamber wall fills with blood which stagnates, promoting clots. The scientific literature (e.g. Ogden) reports such filters actually generate more clots than they trap.
3. Assembly is difficult and costly because the filter must be inserted with enough force to surmount the detents.
Disadvantages of injection molded chambers and top caps include:
1. Leaks between the top cap and the chamber body.
2. Clots promoted by blood foaming in its free-fall from a short downspout at flow-rates typically above 250 ml/min. The common name of "drip chamber" came about because at low flow rates the blood "dripped" from a short downspout opening to the blood level. More recently, flow rates of up to 650 ml/min have become possible because of newer hemodialyzers. But short downspouts foam severely at fast flow rates due to turbulence and cavitation caused by the rapid, continuous free fall of blood. Foam leads to increased clots due the greater surface area of an air/blood interface.
3. Clots promoted by blood stagnating in the space between long downspouts and the chamber wall. Long downspouts with an outlet beneath the blood level have been developed more recently in an attempt to reduce turbulence, in the same manner a hose dipped into a bucket of soapy water will not foam. However, blood stagnates in the tight space between the downspout and the chamber wall. Stagnation frequently leads to clots.
4. Clots promoted by blood stagnating above the outlet of a long downspout. At slower flow rates, long downspouts are counterproductive because blood stagnates in the blood layer above the downspout outlet, resulting typically in clotting.
5. Excessive air/foam alarms at high flowrates with long downspouts. Entrained air bubbles in the incoming blood flow is projected far enough down into the chamber such that they are sensed by the air/foam detector, creating many alarms and stopping the blood pump. It is well known that clotting increases whenever the blood flow is stopped.
6. Injection molding typically provides a structure having fairly sharp angles and edges, which is generally undesirable in blood handling apparatus.